Abstract
Abstract A modified first-order model is proposed for scalar diffusion in the convective boundary layer. In addition to the eddy diffusivity term, the model includes a term proportional to the second derivative of the mean scalar. The coefficient of the new term is closely related to the skewness of the vertical velocity component and to the inhomogeneity of the turbulent field. The model is used to explain the difference in eddy diffusivity between the top-down and bottom-up diffusion as well as the countergradient transport of the bottom-up scalar. The model is different from the approach of top-down and bottom-up decomposition in the respect that the former does not assume a linear profile of scalar flux or does not depend on the ratio of the entrainment flux to the surface flux. Unlike a previous countergradient term, the proposed term does not need to be combined with the top-down and bottom-up decomposition. The model equation is numerically solved to obtain the scalar gradient profile for three typ...
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